Automatic telephone system



Aug. 20, 1929. c. E. L oMAx AUTOMATIC TELEPHONE SYSTEM original FiledJune 14, 1924 3 Sheets-Sheet Imren nr EZarEncE E. Lumax Aug. 20, 1929.c, E. LoMAx AUTOMATIC TELEPHONE SYSTEM s seets-sheet Original Filed June14, 1924 Imran ur mal-'Enns E'. I.. Umax Aug. 20, 1929.

C. E. LOM AX AUTOMATIC TELEPHONE SYSTEM Caz" Enns ELnmax Patented Aug.20, 1929.

UNITED STATES PATENTOFFICE.

CLARENCE E. LOMAX, OF CHICAGO, ILLINOIS, ASSIGNOR, BY MESNE ASSIGNMENTS,TO

AUTOMATIC ELECTRIC INC., OF CHICAGO,

WARE.

ILLINOIS, A CORPORATION OF DELA- AUTOMATIC TELEPHONE SYSTEM.

Application filed .Tune 14, 1924, Serial No. 719,964. Renewed May 5,1928.

This invention relates in general to automatic telephone systems and itsobject, broadly stated, is to provide a new and improved arrangement oftrunking using twenty level selectors and connectors as overflowswitches.

More specifically, the object of the present invention is the provisionof a trunking a1'- rangement whereby a predetermined number of the lasttrunks of two of the same or dif ferent groups of switches areterminated in the same group of overflow switches. The overflow switchesare of the twenty level type with two sets of wipers; the set to be usedbeing determined by the level over which the call comes from thepreceding switch.

The above pointed out object, together wit-h others not specificallymentioned at this time, will be pointed out and explained more fullyhereinafter, reference being had to the accompanying drawings, in which-Fig. 1 is a circuit diagram of. an automatic substation A, a rotary t peline switch C, an overflow selector E o the Strowger type, and theoutline of a regular selector D also of' the Strowger type,

F ig. 2 is a circuit diagram of an overflow connector H of the Strowgertype, a rotary type line switch C, and an automatic substation A, s

Vhile Fig. 3 is a schematic diagram of the trunking arrangement.

The general layout of the system will first be explained by means of thetrunk diagram Fig. 3. This is a schematic drawing and is intended toshow in simple form the trunking between groups of first selectors.lsecond selectors and connectors in an automatic exchange involving theinvention. For example, trunks 1-7, inclusive, of level 1 of group #1 offirst selectors, such as D, extend to a group of seven regular secondselectors, such as E3, in the first thousand. Trunks 8-10, inclusive, ofthis same level and group extend to a group of three overflow secondselectors,

such as E. Trunks 1-7, inclusive, of level 2 of group :#:1 of firstselectors extend to a group of seven regular second selectors, such asE', in the second thousand; while trunks 8-10, inclusive, of this samelevel extend to the previously mentioned group of three overflow secondselectors. The outgoing trunks of levels 1 and 2 in group #2 of firstselectors, such as D', are similarly extended to groups of secondselectors, such as E4, E2,

and E; trunks 8-10,A inclusive, of both levels being multipled withcorresponding trunks of levels l and 2 in group #1.

The overflowV selectors, such as E, have two sets of ten level banks andwipers, the lower set being in the first thousand group of' secondselectors, while the upper set is in the second thousand group. The setof wipers and banks to be used depends upon the level over which thecall comes from the first selectors, as will be more fully explainedhereinafter in connection with the operation of the apparatus. 1

Trunks 1-3, inclusive, of level 1 of the lower' set of banks of thegroup of overflow selectors are multiplied with trunks 1f-6, inclusive,of level 1 of the group of regular selectors E, and together with trunks1-3, inclusive, of the latter level and group of switches extend to agroup of six regular connectors, such as H4 in the first hundred. Trunks4 6, inclusive, of level 1 of the lower set of banks of' the group ofoverflow selectors E are multipled with trunks 4 6. inclusive. of level1 of the group of regular selectors E3 and together with trunks k3,inclusive, of the latter' level and group of switches extend to a groupof six regular connectors, such as H3. Trunks 7-10, inclusive, of level1 of the lower set of banks of the group of overflow selectors E aremultiplied with corresponding trunks of level 1 in the group of regularselectors E3 and E4 and extend to a group of four overflow connectorssuch as H.

The outgoing trunks of level 2 of the lower set of banks of the group ofoverflow selectors E, the group of regular selectors E3 and the groupregular selectors E4 in the first thousand are similarly multipled andextended to groups of overflow connectors H and regular connectors H andH2 in the second hundred.

The outgoing trunks of levels 1 and 2 of the upper set of' banks of thegroup of overflow selectors E and the outgoing trunks of levels 1 andA 2of. the groups of regular selectors E and E2 in the second thousand aresimilarly multipled and extended to other groups (not shown) of overflowand regular connectors in the first and second hundreds.

The overflow connectors H have two sets of wipers, the same as overflowsecond selectors E, one set functioning with the lower ten levels whichare in the first hundred group;

while the other set functions with the upper ten levels which are in thesecond hundred group. The set of wipers to be used depends upon thelevel over which the call comes from the second selectors, as alreadymentioned in connection with a call coming in to the twenty leveloverflow second selectors from first selectors. The wiring of only twolevels has been indicated throughout, but it will be understood that thesame general plan may be followed with levels 3 to O, inclusive, orthese other levels may be connected in the usual manner, omitting theoverflow selectors, depending on the circumstances.

Having ex alained the trunking arrangement adopted, the apparatus shownin the circuit diagrams may now be briefly cxplained. Referring to Fig.1, the substation A is an ordinary automatic telephone substation havingthe usual talking instrumentalities and a calling device forlcontrolling the automatic switches. The line conductors terminate atthe exchange in the individual line switch C, which is of the well knownrotary type. The first selector D, represented by a rectangle, ismechanically of the usual and well known Strowger vertical and rotarytype. The overflow second selector D is mechanically of the usual andwell known Strowger vertical and rotary type except that it has two setsof wipers and a twenty level bank. Besides the usual relays and magnetsthis switch has an additional switching` relay which allows the normallyconnected wipers to be used, or switches to the wipers not normallyconnecteddepending upon which level of the first selector the callycomes in over.

Referring now to 2, the overflow connector H is mechanically of theusual andV well known Strowger vertical and rotary type, but also hastwo sets of wipers and a twenty level bank. Besides the usual relays andmagnets, this switch has an additional switching relay which allows thenormally connected wipers to be used or switches to the wipers notnormally connected, depending upon which level of the second selectorthe call comes in over. The twenty level bank is divided into two units,one for each set of wipers, and each unit has lines extending tosubscribers stations in different hundred groups. A set of line contactsin the level of the lower bank, as shown, extend to the subscribersstation A, which is similar to A, by way of rotary line switch C', whichis similar to C.

Having briefly explained the purpose of v the invention and theapparatus involved in carrying it out, the operation of the equipmentmay now be explained more in detail. For the purpose it will be assumedthat the subscriber at substation A desires to obtain connection withthe subscriber at substationu A, accessible by way of the first selectorD, twenty level overflow second selector E, and

twenty level overflow connector H. In order to do this, the subscriberat substation A removes his receiver and then dials the directory numberof the desired subscriber` W hen the receiver` is removed at substationA, line relay 14 energizes over line conductors 11 and 12. Uponenergizing, line relay 14 places ground on private normal conductor 30at arniiature 20, thereby making the calling line busy, and closes aci"cuit for switching relay 13 and stepping magnet 15 in series. As afurther result ofthe energize.- tion of line relay 11i, test wiper 22 isconnected to the junction of switching relay 13 and stepping magnet 15at armature 19.

In case the trunk upon which the wipers of the line switch are standingis idle, switching relay 13 energizes immediately. Assuming the trunk tobe busy, however, switching relay 13 is short circuited by groundencountered on the busy test contact by test wiper 22 and does notenergize, and the wipers are rotated step by step by the buzzer-likeaction of stepping magnet 15 which operates through test wiper 22.ViThen an idle trunk is reached, which trunk, it will be assumed, is theone terminating in bank contacts 2l-26, inclusive, switching relay 13,being no longer short circuitcd, energiZes in series with steppingmagnet 15. Stepping magnet 15, however, does not energize at this timeon account of the high resistance of switching relay 13. Switching relay18, upon energizing, opens a point in the test circuit and preparesthe'holding circuit at armature 17 and at armatures 16 and 18disconnects line conductors 11 and 12 from the winding of line relay 14and ground, and extends them by way of the wipers 21 and 23, bankcontacts 24 and 25, to the upper and lower windings of the line relay(not shown) of the first selector D.

The line relay of the rst selector D now energizes over the callingsubscribers loop and closes a circuit for the release relay (not shown).Upon energizing, the release relay places ground on the release trunkconductor, thereby closing the usual holding circuit for switching relay13 of the line switch C before the slow-actin c' line relay 14 has hadtime to deenergize. When the calling subscriber operates his callingdevice in accordance with the iirst digit in the desired number. whichin this case it will be assumed is 1, the first selector D raises itswipers 31, 32 and 33 opposite the first level of bank contacts and thenselects an idle trunk in the'usual well known manner. lt will be furtherassumed that the trunk selected at this time is the one extending to thetwenty level overflow selector E terminating in bank contacts numbered35, 37 and 39.

The line relay l1 of the overflow selector D now energizes over thecalling subscribcrs loop and closes at armature 52 a circuit for releaserelay 42. Upon energizing, release relay 42 opens a point in the circuitof release magnet 46 and prepares a circuit for vertical magnet- 47 atarmature 54, and at armature 53 places ground on the release trunkconductor, thereby closing the usual holding circuit for the precedingswitches D and C.

When the calling subscriber operates his calling device in accordancewith the -second digit in the desired number, which it will be assumedis 1 in this case, the circuit of line relay 41 is opened acorresponding number of times, or once. The line relay 41 deenergizes inresponse to the interruption thereby produced and closes a circuit forvertical magnet 47 as follows: From ground by way of the resting contactof armature 60 and said armature, armature 52 and its resting contact,armature 54 and its working contact, relay 43, and vertical magnet 47 togrounded battery. By the operation of vertical magnet 47, the wipers68-73, inclusive, are raised opposite the first level of bank contacts.Relay 43 is energized in series with vertical magnet 47 and, being slowacting, retains its armature attracted throughout the vertical movement.As a result of the energization of relay 43, it closes a circuit atarmature 55 for stepping relay 44 by way of olf normal contacts 51,which close upon the first vertical step, and interrupter contacts 50.Stepping relay 44, upon energizing, closes a locking circuit for itselfat armature 56 and at armature 57 prepares a circuit for rotary magnet48.

At the end of the vertical movement,- slow acting relay 43 deenergizesand completes the circuit of rotary magnet 48 at armature 55. Rotarymagnet 48, upon energizing, advances the wipers 68-73, inclusive, intoengagement with the first set of bank contacts in the selected level,and, near the end of its stroke, opens the circuit of stepping relay'44at interrupter contacts 50. Stepping relay 44, upon deenergizing, opensthe circuit of rotary magnet 48 at armature 57, whereupon rotary magnet48 deenergizes also and closes its interrupter contacts 50 again. Theupper bank wipers 68, 69 and 70 are not now connected to the selector'E. In case the trunk terminating in the first set of the lower bankcontacts is idle, switching relay 45 energizes immediately. Assuming thetrunk to be busy, however, switching relay 45 is short circuited by theground potential encountered by test wiper 72 and stepping relay 44energizes from this same ground potential and closes the circuit ofrotary magnet 48 again, whereupon the switch wipers are rotated intoengagement with the next set of bank contacts. This alternate operationof stepping relay 44 and rotary magnet 48 continues until an idle trunkis reached, which trunk, it will be assumed, is the one terminating inbank contacts 81, 83 and 85, whereupon switching relay 45, being nolonger short circuited, energizes in series with stepping relay 44.Stepping relay 44, however, does not energize at this time on account ofthe high resistance of switching relay 45. Switching relay 45, uponenergizing, opens a point in the test circuit and prepares the holdingcircuit at armature 59; disconnects ground from arma ture 52 of linerelay 41 atarmature 60, thereby opening the circuit of release relay 42;and at armatures 58 and 61 disconnects the line conductors from thewindings of line relay 41 and extends them by way of wipers 71 and 73,bank contacts 81 and 85, and the associated trunk conductors to thewindings of the line relay of the twenty level overflow connector H.

The circuit of the line relay 111 of the connector H may be traced fromground, lower winding of relay 111, normally closed contact controlledby armature 109, lower winding of relay 144, conductor 89, the callingsubscribers loop by way of selectors E and D and line switch C,conductor 86, normally closed contact controlled by armature 108, andupper winding of relay 111 to grounded battery. Relays 111 and 144energize in series over this circuit; the former closing a circuit forthe release relay 112, which, upon energizing, prepares the connectorfor operation in its vertical movement inthe usual way and at armature125 connects ground to the release trunk conductor. A holding circuit isthus established which maintains all the previously operated switches inoperated position in the usual manner. Relay 144 disconnects the wipers147, 148 and 149 from and connects the wipers 150, 151 and 152 to theconnector H at armatures 141, 142 and 146, respectively; closes alocking circuit for itself at armature 143 by way of ground at armature125; and closes at armature 145 a shunt for its lower winding, removingsame from the talking circuit.

When the next impulse, it being assumed that a one is dialled, istransmitted over the control circuit by the calling subscriber, the linerelay 111 of the connector H is momentarily deenergized once and thisdeenergization transmits an impulse of current to the vertical magnet118 over path which extends from ground by way of armature 124 and itsresting contact, armature 126 and its working contact, off normalsprings 121 and 123, winding of the slow acting series relay 113, andthe winding of the vertical magnet 118 to grounded battery. By theoperation of the vertical magnet, the wipers 147-152, inclusive, areraised one step until they stand opposite the first level of bankcontacts. Relay 113 is energized in series with the vertical magnet 118and, being slow acting, holds up continuously during the verticaloperation of the switch. By the operation of this relay, the verticalmagnet circuit is preserved intact notwithstanding the shifting of theoff normal springs which occurs at the first vertical step ing contact,oft normal springs 121 and 122,

armature 127 and its resting contact, resting contact of armature 132and said armature, armature 139 and its resting Contact, and the windingof the rotary magnet 119 to grounded battery. By the operation of therotary magnet, the wipers 147-152, inclusive, are rotated step by stepand are finally brought to rest in engagement with the particular set ofbank contacts in which the line of substation A is terminated, thesecontacts being 4indicated in the drawings by the reference characters156, 157, and 158. The slow arcting relay 116 is energized in parallelwith the rotary magnet and remains continuously energized during therotary movement. In operated position, relay 116 connects the test wiper151 to the winding of test relay 115 by means of its armature 133 and atarmature 135 closes an alternate point in the circuit of the rotarymagnet to guard against the possibility or' having this circuit openedby the operation of the test relay while the test wiper 151 is passingover grounded test contacts.

The final digit has now been received and the connector wipers have beenplaced in connection with the terminals of the called line. If the lineis busy, there will be a ground potential on the test contact 157 andthe test relay 115 will be energized. Then this relay operates, itprepares a locking circuit for itself at its armature 131 which iscompleted when the slow acting relay 116 falls back an instant later,opens the rotary magnet circuit at armature 132, and at armature 134connects a lead from the busy signalling machine to the lower side ofthe line. On perceiving the signal, the subscriber will replace hisreceiver, thereby releasing the connector H and all previously operatedswitches.

Assuming now the called line is idle when connection therewith isattempted, under these circumstances there will be no ground on the testcontact 157 and the test relay 115 will not be energized. Then, when theslow acting relay 116 falls back, a circuit is completed for theswitching relay 117 which may be traced from the grounded conductor 97by way of armature 131 and its rest-ing Contact, the lower winding ofthe switching relay 117, resting contact of armature 133 and the saidarmature, test wiper 151, test contact 157, switching relay 170 of linesw'itch C', and the stepping magnet 172. Upon the closureof this circuitrelays 117 and 17 O are energized in series and the latter relay isoperated to disconnect the line of substation A from its normal batteryand ground connections in the line switch. The relay 170, however', isoperated only about half way due to a mechanical interlocking ldevicecontrolled by line relay 171, and the line conductors are not connectedthrough to the line switch wipers.

In the connector H, when the switching relay 117 pulls up, itestablishes a locking circuit for itself at armature 138, opens therotary magnet circuit at armature 139 and at armatures 136 and 140connects up the two line wipers 150 and 152. By the latter operation, asignalling circuit is established, whereby ringingcurrent from thegenerator GEN is intermittently projected out over the called line tooperate the bridged ringer at the substation A to notify the calledsubscriber that he is wanted. The return path for the ringing currentincludes the upper winding of ring cut-olip relay 114, and, when thecalled subscriber answers, this relay is operated. Upon energizing,relay 114 establishes a locking circuit for itself at armature 129,breaks the ringing circuit at armatures 128 and 130 and at the workingcontact of these same armatures finally completes the talkingconnection.

Current is now supplied to the transmitter at substation A through thewindings of the double wound back bridge relay 110, which is accordinglyenergized. This relay is a reversing relay and has the well knownfunction of reversing the incoming trunk conductors 86 and 88 or 89,depending upon which one is used, as regards their connections with thewinding of relay 111. This reversing operation is a standard operationand is for the purpose of operating a subscribers pay-station ormetering apparatus and in some cases is used for supervisory purposes.The subscribers may now converse as desired.

When the subscribers are through talking they replace their receivers.The replacing of the receiver at the called substation does not produceany particular results, except that current merely stops flowing overthe calledrline and relay 110 deenergizes to reverse the current to thecalling subscriber as previously explained.

When the receiver is replaced at substation A, the circuit of line relay111 is opened and this relay deenergizes. Upon armature 124 beingretracted, the circuit of release relay 112 is opened and a circuit isprepared for release magnet 120. Upon deenergizing in response to theprolonged opening of its circuit, slow-acting release relay 112 removedground from armature 131 of relay 115 at armature 149; disconnectsground from release trunkconductor 87 at armature 125, .whereupon relays114, 117 and 144 deenergize; and at armature 126 completes a circuit forrelease magnet 120 by way of off normal contacts 107. Upon theenergization of release magnet 120, the wipers of the connector H arerestored to normal position, and the circuit of release magnet 120 isopened at olf normal contacts 107 by the switch shaft when the wipersreach their normal position. At the same time the wipers are restored tonoimal, switching relay 144 opens the circuits of the -lower bank wipersand prepares the circuits of the upper bank wipers at armatures 141, 142and 146, thus restoring the circuits of the Wipers to normal conditions.

As a result of the ground potential being removed from release trunkconductor 87, switching relay 13 of the line switch C, the switchingrelay ,.(not shown) of the ordinary selectorI D, and the switching relay45 of the overflow selector E deenergize. rlhe deenergization ofswitching relay 13 of the line switch C restores conditions in thatswitch to normal. The ordinary selector D is restored to normal in amanner similar to that of overflow selector E which will now beexplained. The deenergization of switching relay 45 of the overflowselector E results in a circuit being closed for release magnet 46 atarmature 60 and its resting contact. This circuit includes armatures 52and 54 and their resting contacts and oft normal contacts 49. The wipersof the selector E are restored t0 normal position in the usual mannerupon the energization of release magnet 46, and the circuit of releasemagnet 46 is again opened at off normal contacts 49 when the switchwipers reach their normal position.

In order to explain the operation a little more in detail, it will beassumed that the first digit dialled is 2 instead of 1. The selector Draises its wipers 31, 32 and 33 opposite the second level of bankcontacts and then selects an idle trunk. It will be assumed that thetrunk selected at this time is the one whose bank contacts are numbered34, 36 and 38, extending to the overflow selector E. That being thecase, line relay 41 and switching relay of the overflow selector Eenergize in series over the calling line, the circuit being traceablefrom ground, lower winding of relay 41, resting contact of armature 61and said armature, lower winding of relay 65, the calling subscribersloop by way of selector D and line switch C, armature 58 and its restingcontact, and upper winding of relay 41 to grounded battery. Relay 41closes a circuit for the release relay 42, which upon energizingprepares the selector for operation and connects ground to the releasetrunk conductor in the usual manner. Relay 65 disconnects the wipers 71,72 and 73 from and connects the wipers 68, 69 and 7 0 to the overflowselector E at armatures 62, 63 and 67, respectively; closes a lockingcircuit for itself at armature 64 by way of ground at armature 53; andcloses at armature 66 a shunt for its lower winding. The remainingoperations are similar to those previously described except that theconnection is extended via wipers 68-70, inclusive, instead of wipers71-73, inclusive.

It will now be assumed that the subscriber at station A dials 1 as thefirst digit and 2 as the second digit. The first selector D raises itswipers to the lirst level and selects an idle trunk, which trunk it willbe further assumed is the one terminating in the overflow selector E.The overflow selector E raises its wipers to the second level andselects an idle trunk, which trunk it will be assumed is the onenumbered 86, 87 and 88 and terminating in the overflow connector H. Vhenthe connection is extended to the connector H the line relay 111energizes over the subscribers loop and performs the functions in theusual well known manner; however, the switching relay 144 is not nowconnected in series with the line relay and therefore does not energizeto switch the wiper circuits at armatures 141, 142 and 146. Except forthis the operations are the same as previously described.

The principal advantage in a system similar to the foregoing lies in thefact that the well known graded multiple scheme of trunking betweengroups is extended to include levels by using twenty level overflowswitches, and since the economy of all graded multiple schemesis due tothe fact that the peak load in all groups does not occur at the sametime, the economy of the twenty level overflow switches used ashereinbefore described is apparent due to the fact that the peak loadfrom two levels of the same switch or group of switches does not occurat the same time. It is recognized that the twenty level switch is notan economy for ordinary use due to the fact that a single call willcause a large number of bank contacts to be idle, but in this schemeonly a small percentage of large banks are used and the saving effectedby more eilicient trunking more than offsets the use of the few largebank switches.

Having described my invention, what I consider to be new and desire tohave protected by Letters Patent will be pointed out in the appendedclaims.

What is claimed is:

1. In a telephone system, first selectors, a group of second selectorsaccessible from one level of said first selectors, a second group ofsecond selectors accessible from another level of said first selectors,a group of overflow selectors accessible from both said levels, saidoveriiow selectors having two sets of banks, the bank contacts of eachset being grouped separate from the bank contacts of the vother set,multiple connections between one set of the overllow selector banks andthe banks of the iirst group of second selectors, and multipleconnections between the other set of overflow selector banks and thebanks of the other group of second selectors.

2. In a telephone system, selectors, a group of connectors accessiblefrom one level et' said selectors, a second group of connectorsaccessible Jfrom another' level of said selectors, a group of overflowconnectors accessible from both said levels, said overflow connectorshaving two sets of banks, each of said overflow connectors having aseparate set of wipers for each set of banks, multiple connectionsbetween one set oi' overflow connector banks and the banks of said firstgro-up ot' connectors, and multiple connections between the second setof overfiow connector banks and the banks of the other group ofconnectors.

3. In a telephone system, a group of selectors, two groups of trunksaccessible to said selectors, two groups of second selectors in whichthe said two groups of trunks terminate, respectively, except tor arelatively small plurality of trunks in each group, a group of overflowselectors in which said small pluralities of trunks terminate, and banksfor said overflow selectors comprising two distinct sections multipledwith the banks of said two groups et second selectors, respectively.

4. In a telephone system, two groups of switches, each switch having asingle set of wipers and banks, a group of overflow switches, eachoverflow switch having two sets of wipers and banks, and multipleconnections between the two sets of overflow switch banks and the bankso'i said two groups of switches, respectively.

5. In a telephone system, two groups of first selectors, four groups ofsecond selectors accessible from two levels of said two groups of firstselectors, and a single group of overflow selectors accessible from bothlevels of both groups of first selectors.

6. In a telephone system, three groups of selectors, connectorsaccessible only to selectors of the first group, other connectorsaccessible only to the selectors ol the second group, other connectorsaccessible only to selectors ofthe first and third groups, otherconnectors accessible only to selectors of the second and third groups,and other connectors accessible to selectors of all three groups.

7. An automatic switch having two sets of wipers, each set comprisingtwo line wipers and a test wiper, two trunk lines extending to saidswitch, a line relay in said switch energized when said switch isconnected with via either trunk line, and a`wiper selecting relayenergized in series with said line relay only when said switch isconnected with via a particular one of said trunk lines.

8; An automatic switch having two sets of wipers, two trunk linesextending to said switch, a line relay in said switch energized whensaid switch is connected with via either trunk line, by current flowover the two sidi-'s of the concerned trunk line in series, a wipt-rselecting relay energized in series with said line relay only when saidswitch is connected with via a particular one of said trunk lines, and alocking circuit for maintaining said wiper selecting relay energizedindependent of the trunk line over which it is initially energized.

9. An automatic switch having two sets of wipers, two trunk linesextending to said switch, a line relay in said switch energized whensaid switch is connected with via either trunk line, a wiper' selectingrelay energized in series with said line relay only when said switch isconnected with via a particular one of said trunk lines, said wiperselecting relay having a winding for initially energizu ing it as setforth, and a second winding, a locking circuit including said secondwinding, and contacts on said wiper selecting ref lay for shortcircuit-ing its initial energizing winding.

10. In a telephone system, a group of first selectors, a group of secondselectors, each second selector being accessible in a plurality oflevels in the banks of said first selectors, two sets of banks for saidsecond selectors, two groups of connectors, each connector in one groupbeing accessible in a plurality of levels in one set of banks of saidsecond selectors, and each connector in the other group -beingaccessible in a plurality of levels in the other set of banks of saidsecond selectors.

11. In a telephone system, three groups of switches of one switchingstage, switches of the succeeding switching stage accessible only toswitches of the first of said three groups, other lswitches of thesucceeding switching stage accessible only to the switches of the secondof said three groups, other switches of the succeeding switching stageaccessible only to the switches of the first and the third of saidgroups, other switches of the succeeding switching stagey accessibleonly to switches of the second and the third of said groups, and otherswitches ofthe succeeding switching stage accessible to switches of allthree of said groups.

12. In a telephone system, two groups of switches, each switch of thesecond group being accessible to each switch ofthe first group in aplurality of levels, two sets of banks for each switch of the secondgroup, a third and a fourth group of switches, each switch of the thirdgroup being accessible in a plurality of levels in one set of banks ofeach switch of the second group, and each switch of the fourth groupbeing accessible in a plurality of levels in the other set of banks ofeach switch of the second group.

13. An automatic switch having two sets of wipers each set comprisingtwo line wipers and a test wiper, two trunk lines extending to saidswitch, a line relay in said switch energzed by current flow over thetwo sides of the trunk line in series when said switch is connected withvia either trunk line, a wiper selecting relay energized in series withsaid line relay only when said switch is connected with via a particularone of said trunk lines, and a locking circuit for maintaining saidwiper selecting relay energized independent of the trunk line over whichit is initially energized.

14. An automatic switch having two sets of Wipers each set comprisingtwo line wipers and a test wiper, two trunk lines extending to saidswitch, a line relay in said switch energized when said switch isconnected with via either trunk line, a wiper selecting relay in saidswitch energized in series with said line relay only when said switch isconnected with via a particular one of said trunk lines, a winding ofsaid wiper selecting relay for initially energizing it as set forth, asecond winding of said wiper selecting relay, a locking circuit for saidrelay including said second winding, and contacts on said wiperselecting relay for short circuiting its initial energizing winding.

15. An automatic switch having two' sets of wipers each set comprising aplurality of line wipers and a test wiper, two trunk lines extending tosaid switch, a line relay in saidswitch energized by current flow overboth sides of the trunk line in series when said switch is connectedwith via either trunk line,

a wiper selecting relay in said switch energized in series with saidline relay only when said switch is connected with via a. particular oneof said trunk lines, a winding of said wiper selecting relay forenergizing it initially as set forth, a locking circuit for said wiperselecting relay independent of said initial energizing winding, andmeans responsive to the energization of said wiper selecting relay forclosing said locking circuit and for short circuiting said initialenergizing winding.

16. In a telephone system, three groups of switches of one switchingstage, a group of trunks accessible in the corresponding levels of thebanks of said switches and terminating in switches of the succeedingswitching stage said group of trunks providing the only means of accessfrom said levels to switches of the succeeding switching stage, aportion of said group of trunks accessible only in the banks of theiirst and third groups of switches of said first switching stage,another portion of said group of trunks accessible only in the banks ofthe second and third groups of switches of said first switching stage,and another portion comprising the balance of said group of trunksaccessible in the banks of all three groups of switches of said firstswitching stage.

In witness whereof, I hereunto subscribe my name this 12th day of June,A. D., 1924.

CLARENCE E. LOMAX.

